Midthoracic sympathetic preganglionic axons make a choice to either ascend rostrally or descend caudally along the paravertebral chain of sympathetic ganglia; preganglionic axons do not bifurcate to project in both directions even though the segment from which they derive provides innervation to both rostral and caudal ganglia. There is both an intersegment and an intrasegment positional dependence to the choice made. The intersegmental positional dependence is reflected by the fact that nearly all of the preganglionic axons from rostral thoracic segments project rostrally, while those from caudal thoracic segments project caudally. There is a smooth gradient from predominately rostral projections to predominately caudal projections along the preganglionic cell column. There is also a prominent intrasegmental gradient of positional dependence in midthoracic segments: preganglionic neurons located rostrally within a segment project rostrally and those located caudally project caudally. This proposal presents a series of experiments designed to illuminate the mechanisms that result in the segment-specific preganglionic axon projections. The specific aims are: 1. To determine whether the somites influence the choice of axonal trajectory by preganglionic neurons. 2. To determine whether sympathetic preganglionic neurons in the thoracic spinal cord are intrinsically segmented. 3. To determine whether a morphogenetic gradient susceptible to retinoic acid underlies the intersegmental positional dependence of axon trajectory. 4. To determine whether rostrally and caudally projecting preganglionic axons are grouped according to either lineage restrictions or selective fasciculation. 5. To determine whether environmental boundaries or facilitating substrates influence the choice of axonal trajectory by preganglionic neuro . All of these aims will be accomplished in the chicken embryo because it is amenable to embryonic manipulation. The organization of the avian sympathetic nervous system is quite similar to that in the mammal; thus these experiments will provide information that is generally applicable to sympathetic neuronal development.